Abstract
In the deep space exploration activities, the spacecraft would travel through the interplanetary space and arrive at other planets. The vehicles would be exposed to the harmful space environments during traveling or while orbiting the planet. The radiation environment is one of the most important environmental factors that would affect the spacecraft performance. The radiation environment in the interplanetary space, as well as on Jupiter, Saturn and Mars is described in this paper. Radiation in the interplanetary space consists of galactic cosmic rays and solar protons. Because Jupiter has the strongest magnetic field in the Solar system, the particle energy in its radiation belt is ten times that of the Earth’s radiation belt and its flux is a few orders of magnitude larger than at the Earth. In this harmful radiation environment, many of the materials used in the vehicle design would suffer significant properties degradation or failure. Also the effects of these severe radiation environments on vehicle materials are analyzed in this paper.
Keywords
Introduction
The deep space exploration has never been stopped. So far, the exploration activities conducted on the solar celestial bodies beyond the moon have exceeded 200 [1, 2]. When the deep space exploration spacecraft is travelling to the target celestial body or orbit around the target, it will encounter the harsh space radiation environment. One of the main features of deep space beyond the Earth’s magnetosphere is a radiation field of energetic particles. The radiation environment experienced by a deep space explorer can be divided into three stages: first, the radiation environment during the journey from the Earth to other planets, its main source of radiation is solar particle events and galactic cosmic rays (GCR). Second, the radiation environment during landing on the target planet, its main source of radiation is energetic particles trapped by the planet magnetic field; third, the surface radiation environment of deep space planets, it is mainly from the second radiation due to the interaction of the planet body and the cosmic rays. These radiation environments are mainly composed of energetic particles.
This radiation environment will cause the sensitive materials and devices onboard the vehicle a more serious degradation than in the Earth’s orbit. In the long term exposure to a deep space environment, the vehicle materials, such as thermal control materials, solar cells, optical materials, insulating materials, sealing materials and alike, encounter a severe degradation, including deterioration of optical properties, electrical properties and mechanical properties, as well as degradation of insulating performance and sealing performance.
This paper reviews the investigation of the planet’s radiation environments and their models during deep space exploration activities, and provides a preliminary analysis on the degradation effects of deep space environment on spacecraft materials.
Deep Space Radiation Environment
Interplanetary Space Radiation Environment
The radiation environment in the interplanetary space is mainly composed of solar particle events and galactic cosmic rays. The galactic cosmic rays are charged particles from outside of the solar system, their main component are high energy protons, alpha particles and heavy nuclei with an atomic number greater than 2 [2]. The occurrence and intensity of solar particle events related to the solar activity cycle, consisting mainly of high energy protons, in addition to a small amount of alpha particles and heavy nuclei.
Radiation Environment at Mars
Martian surface radiation environment mainly includes galactic cosmic rays, occasional solar proton events and secondary neutrons. The galactic cosmic rays reaching Mars’ surface is one of the main sources of radiation. Figure 1 shows the absorbed dose in solar maximum (1990) and solar minimum (1977) under different shielding materials and thickness. The Mars surface radiation environment by Marsgram atmosphere model during solar maximum (1990) and solar minimum (1977) activities is shown in Fig. 2.
Mars surface also is affected by the solar particle event (SPE). However, due to attenuation in the Mars atmosphere, the SPE dose reaching Mars’ surface decreases approximately one order of magnitude compared to the SPE dose in space. Figure 3 shows the SPE dose rate at Mars’ surface measured by the Marie detector and its variation with orbit time, with the peak dose rate of 60 mrad/day [3].
Galactic cosmic rays and solar energetic particle will interact with Mars’ atmosphere and low energy charged particles will be absorbed, changing the particle distribution on the surface of Mars. Meanwhile, the particles interact with the Mars’ surface materials and produce secondary neutron radiation reflected back into the atmosphere. The secondary neutron component is closely related to the surface of Mars. On the regolith and bedrock, neutrons with energy below 20 MeV diffused into atmosphere from the surface is in absolute dominance in the Mars’ surface neutron environment. Thus, the flux and energy of surface neutrons vary for different surface and different atmospheric conditions.
Radiation Environment on Jupiter
Jupiter is located on an orbit in 5.2 AU from Sun. It has at least 60 satellites, four main satellites are Io, Europa, Ganymede and Callisto. Jupiter has a more powerful magnetic field than Earth. Its core magnetic field is 17,500 times stronger than the Earth’s core. On the surface of Jupiter, the magnetic field strength becomes smaller, but it is still more than ten times stronger than the Earth’s surface magnetic field. Jupiter’s magnetic moment is 1.55 × 1020 Tm3, approximately 20,000 times higher than the Earth’s magnetic moment.
Because Jupiter has the strongest magnetic field in the solar system, its energy density and number of trapped particles is much higher than on other planets. The energy of trapped particles in Jupiter’s radiation belt is ten times higher than the energy of particles in the Earth’s radiation belt, and its number density is several orders of magnitude more than the Earth’s radiation belt. Now it is considered that most of the radiation belt particles are mainly from Jupiter’s moon Io. Therefore, the most intensive part of the magnetosphere is located in the torus between 5.5 and 8 RJ. The results from the Energetic Particles Detector (EPD) in Galileo shows that the proton number and energy density in the range of 20–25 RJ is higher than those of other particles. The results from Ulysses show that the equatorial omnidirectional proton flux has an exponentially decaying distribution with the distance from the magnetic equator and approximately vertical symmetry. The two-dimensional image of synchrotron radiation from the radio telescope shows that there are two different distributions of high energy electrons (1–100 MeV) in the inner radiation belt. First, radiation intensity at the magnetic equator, with the maximum density at 1–1.5 RJ. Second, the electron density has a wide angular distribution and produces radiation over a wide range of magnetic longitudes. The distribution of energetic electrons is limited by absorption in Jupiter’s ring and by its satellites.
Jupiter’s radiation belt is composed of electrons, protons and heavy ions. The first model of the belt was built in late 1950s and in early 1960s, shortly after the discovery of Jupiter’s radiation belt. This model is based on the theory of Earth radiation belt and ground observation results of Jupiter synchrotron radiation. Later, based on the observation of Pioneer and Voyager vehicles, the first comprehensive model of Jupiter trapped radiation was developed in 1980s and 1990s. Recently, the model of high-energy electron and heavy ions was improved on the basis of observation data by Galileo spacecraft. Figure 4 shows the predicted synchrotron emissions at 1.4 GHz and CML 200 for the modified DM electron radiation distributions for E > 1 MeV. Also during this period, the understanding of Jupiter magnetosphere trapped particles was improved based on a physical model developed in Europe.
Radiation Environment at Saturn
Saturn is located at 9.54 AU from the Sun and has an equatorial radius (RS) of 60,268 km. Saturn was first visited by NASA’s Pioneer 11 in 1979 and later by Voyager 1 and 2. Cassini (a joint NASA/ESA project) arrived on July 1, 2004 and orbited Saturn for at least 4 years.
Saturn’s magnetic field is mainly composed of three parts; the first part is the intrinsic dipole magnetic field, the second is the toroidal plasma magnetic field around the Saturn, the third part is from the contribution of solar wind on the magnetosphere.
The former two parts are called inner source field and they have axis symmetry and equatorial symmetry. In the South and North Polar Regions of Saturn, the magnetic field strength is about 0.7 Gauss and 0.6 Gauss, respectively.
Due to the presence at Saturn of a strong magnetic field, a trapped particle radiation belt is formed around this planet. Saturn’s radiation belt was first observed by the instruments on Pioneer 11 in September 1979. These results gave evidence that the observed trapped radiation is absorbed by the satellite and by the Saturn rings.
The Saturnian radiation belts did not receive as much attention as the Jovian radiation belts because they are not nearly as intense, the famous Saturnian particle rings tend to deplete the belts near where their peak would occur. As a result, there was no systematic development of engineering models of the Saturnian radiation environment for mission design.
A primary exception is that of Divine (1990) [4]. That study used published data from several charged particle experiments aboard the Pioneer 11, Voyager 1, and Voyager 2 spacecraft during their flybys at Saturn to generate numerical models for the electron and proton radiation belts between 2.3 and 13 Saturn radii. The Divine Saturn radiation model described the electron distributions at energies between 0.04 and 10 MeV and the proton distributions at energies between 0.14 and 80 MeV. The model was intended to predict particle intensity, flux, and fluence for the Cassini orbiter. Divine carried out hand calculations using the model but never formally developed a computer program that could be used for general mission analyses (Fig. 5).
The Effects of Deep Space Radiation Environment on Materials
Radiation Effects on Spacecraft Materials
Thermal control materials are an important component of the spacecraft thermal control subsystem. The commonly used thermal control materials include thermal control paints, second surface mirrors, polymer films, and so on. Thermal control paints are generally composed of pigments and organic or inorganic matrices. These components will degrade under the radiation environment and their properties will change, thus leading to an increase in the solar absorptance.
Polymer materials are widely used on spacecraft. The effect of radiation environment on polymer materials will lead to cross-linking, degradation, change in the unsaturated bond content and free radical production. This will cause the degradation of physical properties and mechanical properties of polymer materials.
Optical materials in the optical systems will suffer degradation of optical properties under the action of radiation environment. Optical materials will experience the following effects when exposed to electrons or protons: (1) Coloration effects. Many optical glasses and crystals will generate some additional spectral absorptance, i.e. a coloration phenomenon. Materials coloration can lead to a transmittance decrease. (2) Surface erosion. Particles with energies of tens of keV can cause the atoms in the materials composition to be sputtered, leading to surface sputter erosion of materials. In addition to direct sputtering effect, the effect of radiation can also cause foaming at the surface of optical materials, i.e. the optical surface erosion. (3) Charging and discharging effects, energetic charged particles can cause significant surface non-uniform charging, causing the breakdown of glass lenses.
The Effect of Deep Space Radiation Environment on Materials
Due to different deep space exploration missions, spacecraft experience different radiation environments. For more harsh radiation environments of Jupiter and Saturn, we must pay attention to choice of candidate materials. The particle flux of Jupiter’s radiation belt is several orders of magnitude higher than the Earth’s belt. In medium or high Earth’s orbits, the average dose absorbed by surface materials is about 109 rad, thereby we can estimate that the surfaces of materials on spacecraft flying in Jupiter’s radiation belt will absorb a dose more than 1010 rad. Due to such a dose, many polymer materials, such as polypropylene, POM, PTFE, etc., will experience strong performance degradation. Most of optical glasses will be unusable after exposure to 106–108 rad. Thus, we must carry out a detailed analysis of radiation environments met in deep space exploration missions, to determine the candidate materials tolerant to radiation.
Summary
Radiation in the interplanetary space consists of galactic cosmic rays and solar protons. The Martian surface radiation environment mainly includes galactic cosmic rays, occasional solar proton events and secondary neutrons. Because Jupiter has the strongest magnetic field in the Solar system, the particle energy in its radiation belt is ten times higher than that of the Earth’s radiation belt and its flux is a few orders of magnitude larger than the radiation flux on Earth. In this harmful radiation environment, many of the materials used in the vehicle design would suffer significant properties degradation or failure. Thus, we must carry out a detailed analysis of radiation environments met in deep space exploration missions, to determine the candidate materials tolerant to radiation.
References
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Ding, Y., Shen, Z. (2017). Investigation of the Radiation Environment in Deep Space and Its Effect on Spacecraft Materials Properties. In: Kleiman, J. (eds) Protection of Materials and Structures from the Space Environment. Astrophysics and Space Science Proceedings, vol 47. Springer, Cham. https://doi.org/10.1007/978-3-319-19309-0_46
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